Women who were obese during pregnancy were about 67 percent more likely to have autistic children than women who were normal weight. They also had double the risk of having a child with another developmental disorder.
Dr. Irva Hertz-Picciotto of University of California said, "There is a general feeling that some of the origins of autism are probably in the prenatal period when the brain is developing at a very rapid rate."
Diabetic mothers were more than twice as likely to have a child with developmental delays and their children also scored lower on language and communication tests.
Doctors stress this study does not prove obesity and diabetes cause autism but researchers say the results raise serious concerns since more than a third of U.S. women in their childbearing years are obese.
"There are actions that people can take and that's the point. We're trying to find modifiable factors which by and large genetics is not," said Dr. Irva Hertz-Picciotto.
More study is needed to confirm the findings but experts say even if obesity and diabetes are linked to autism, it would be one of many factors that contribute to the disorder.
The Centers for Disease Control now estimates 1 in 88 children have autism in the U.S.